The present application claims priority to Japanese Application(s) No(s). P2000-306876 filed Oct. 6, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a solid electrolyte cell having an electrode unit comprised of a cathode and an anode layered together with the interposition of a solid electrolyte.
2. Description of Related Art
Recently, a variety of portable electronic equipment, such as a video tape recorder with a built-in camera, a portable telephone set or a portable computer, have made their debut. Since it is envisaged to reduce the size and the thickness of these electronic equipment, a demand is raised for reducing the size and the weight of a cell used as a driving power source for the electronic equipment.
As a cell coping with this demand, a non-aqueous electrolyte cell which is made up of a cathode and an anode, capable of reversibly doping or undoping lithium ions, and a non-aqueous electrolyte, and which has advantages such as high output or high energy density, that is a so-called lithium ion cell, was developed and put to practical use.
As the lithium ion cell, there is known a liquid-based lithium ion cell, including, as an ion conductor between the cathode and the anode, a porous high molecular separator impregnated with an electrolyte solution. In such liquid-based lithium ion cell, an electrode unit, comprised of a cathode and an anode, layered together with the interposition of a separator, is housed in a rigid metal can with a view to preventing leakage of the electrolyte solution.
A polymer lithium ion, having a solid electrolyte as an ion conductor between the cathode and the anode, referred to below simply as a solid electrolyte cell, has also been developed. In the solid electrolyte cell, a solid electrolyte, comprised of a lithium salt solid-dissolved in a polymer, and a gelated solid electrolyte, comprised of an electrolyte solution contained in a matrix polymer, is used. Since the solid electrolyte is superior in leakage proofness, it is unnecessary to use a metal. can as an exterior material for housing the electrode unit comprised of a cathode and an anode layered together with the interposition of a solid electrolyte, but a laminate film, for example, may be used. That is, the solid electrolyte cell has an advantage of simplifying the exterior material to reduce its size, weight and thickness.
Meanwhile, if a solid electrolyte cell employing a cathode active material of, for example, LiCoO2, is discharged, the anode potential is changed to noble and reaches the value of 3.8 V (vsLi/Li+), which is the same as the cathode potential, for the cell voltage of 0 V. On the other hand, the eluation potential of Cu, Ni etc used as the anode current collector is lower than 3.8 V (vsLi/Li+), which is the discharge potential of LiCoO2. Thus, in case the anode potential reaches the cathode discharge potential due to overdischarge, the anode current collector, formed by a foil of metal, such as Cu or Ni, is corroded.
If the anode current collector is corroded, the layer of an anode active material may be detached from the anode current collector, or metal ions, such as copper ions, dissolved from the anode current collector into the solid electrolyte, are precipitated during charging on the anode active material. This obstructs lithium intercalation in the anode and the normal anode operation to detract from the cell performance.
If the cell continues to be used under this condition, the solid electrolyte is decomposed during charging where the anode active material is detached on the anode current collector, or where copper is precipitated on the anode active material, so that gases are evolved due to this decomposition reaction. In a solid electrolyte cell in which an electrode unit is hermetically sealed with a laminate film, the gases evolved due to the decomposition reaction of the solid electrolyte are charged into the inside of the laminate film which is thereby swollen to increase the cell size to render it impossible to maintain the shell shape. In particular, if the gas is evolved in an excessive amount, there is a fear of cleavage of the laminate film due to gas charged therein.
Thus, in a solid electrolyte cell, there is provided a protection circuit for prohibiting overdischarge to the cell voltage of 0 V. However, it is up to the solid electrolyte cell to realize a high capacity as the limited cell shape is maintained, such that it is required to remove the protection circuit not contributing to the cell reaction in order to increase the charging volume of the active material. It is similarly requested to remove the protection circuit to reduce the weight of the solid electrolyte cell as well as to reduce its cost.
However, such a solid electrolyte cell in which cell characteristics are not deteriorated even on overdischarge to the cell voltage of 0 V, as the cell shape encapsulated in a laminate film remains unchanged, has not been developed, such that, in the current state of the art, it is not possible to remove the protective circuit from the solid electrolyte cell.
It is therefore an object of the present invention to provide a solid electrolyte cell which can be overdischarged to the cell voltage of 0 V.
The present invention provides a solid electrolyte cell including a cathode containing a compound represented by the general formula LixFe1-yMyPO4 where 0.05xe2x89xa6xxe2x89xa61.2, 0xe2x89xa6yxe2x89xa60.8, and M is at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, an anode, and a solid electrolyte, wherein an electrode unit comprised of the cathode and the anode are layered together with interposition of the solid electrolyte is encapsulated with a laminate film.
In the solid electrolyte cell according to the present invention, the discharge potential of the compound represented by the general formula LixFe1-yMyPO4 is baser than the eluation potential of the metal material of the anode current collector, so that, even when the anode potential reaches the discharge potential of the cathode active material due to over discharge up to the cell voltage of 0 V, the anode current collector formed by a foil of metal, such as Cu or Ni, is not corroded, so that the solid electrolyte cell is not deteriorated in cell performance even on overdischarge, while it is able to maintain the cell shape encapsulated in the laminate film.